How to curve an already made curve?
You can use the powerful Curve modifier to do this. Make a curve to use as the factor, name it something you can remember, and then select the curve you want to bend and go to modifiers and add a Curve modifier. You then will need to select the name of the curve you want to use as the factor, like this:



Now your curve should conform to the shape of the circle, however it may look a little odd:



What you need to do is select the circle and scale it to the desired size, you should end up with something like this: